Systems, devices, and methods for aperture-free hologram recording

ABSTRACT

Systems, devices, and methods for aperture-free hologram recording are described. The apertures typically used for hologram recording create unwanted secondary holograms by diffracting light. Aperture-free hologram recording eliminates these unwanted secondary holograms. Aperture-free hologram recording includes applying a mask to the holographic recording medium. The mask controls the size of the recorded hologram like an aperture but does not create unwanted secondary holograms. Hologram fringes are only present in the desired recording area and a thin boundary region. The mask may be present during recording, or the mask may be used to pre-bleach the holographic recording medium. Pre-bleaching the holographic recording medium renders a portion of the holographic recording medium insensitive to light, the hologram is recorded in the light-sensitive portions of the holographic recording medium.

BACKGROUND Technical Field

The present systems, devices, and methods generally relate to hologramrecording and particularly relate to aperture-free hologram recording.

Description of the Related Art Holograms

A hologram is a recording of a light field, with a typical light fieldcomprising a pattern of optical fringes generated by two beams of laserlight. The hologram is made up of physical fringes, where physicalfringes comprise variations in the refractive index or absorbance of theholographic recording medium.

At least a portion of the light field used to record a hologram may berecreated by illuminating the hologram with laser light. If the laserlight comprises the same wavelength and angle as one of the beams oflaser light used to record the hologram, the holographic medium willemit laser light with the same angle and pattern as the other beam oflaser light used to record the hologram. The intensity of the emittedlight is determined by the efficiency of the hologram, with a higherefficiency hologram emitting more intense laser light. The efficiency ofa hologram depends on both the angle and the wavelength of light used toilluminate the holographic medium. Multiple holograms may be recorded ina single holographic recording medium, the multiple holograms comprisinga multiplexed hologram. A hologram may form a holographic opticalelement (HOE), where the hologram refracts, diffracts, attenuates orotherwise modifies the properties of the light illuminating thehologram.

Hologram Recording

A pattern of optical fringes may be generated by the interference of twobeams of laser light; the two beams of laser light may be created bysplitting a single beam of laser light. The two beams of laser light aretypically referred to as the object beam and the reference beam.Hologram recording is typically designed such that, during playback, therecorded hologram is illuminated with laser light recreating thereference beam and the object beam is then replicated by the hologram.

Holograms are recorded in a holographic recording medium which may be asilver halide photographic emulsion, dichromated gelatin, photopolymer,or other physical media. Silver halide emulsions record a hologram as apattern of absorbance and reflectance of light. Dichromated gelatin andphotopolymer both record a hologram as a pattern of varying refractiveindex. Recording a hologram as a pattern of refractive index isadvantageous since all of the illuminating laser light may theoreticallyleave the hologram; no light is necessarily absorbed by the hologram.

Apertures

A typical hologram recording assembly includes at least one aperture. Anaperture is a device that is placed in the optical path of the laserlight and controls the amount of light that is able to travel furtheralong the optical path. Apertures may be used to control the overallintensity of a beam of laser light; apertures may also control the spotsize of the beam of laser light. In a typical hologram recordingassembly, the physical size of the recorded hologram is determined bythe spot size of the object and reference beams at the holographicrecording medium.

A typical aperture comprises multiple, typically five or six butpossibly more, movable blades. The blades are positioned and orientedsuch that they form an approximate circle with a central gap. The gap ispositioned in the path of the beam of laser light such that at least aportion of the beam of laser light may pass through the gap. Laser lightthat impinges on the blades is blocked and cannot pass any further alongthe optical path. The size of the gap may be varied by moving the bladesrelative to one another. As the size of the gap varies, the size of thebeam of laser light which may pass through the gap varies.

BRIEF SUMMARY

A holographic optical element (“HOE”) comprising a single contiguouslayer of photopolymer material may be summarized as including: arecorded area oriented perpendicular to the principal axis of the HOEwherein the recorded area includes hologram fringes that define at leastone hologram and wherein the hologram fringes comprise a photopolymermaterial with a first amount of refractive index contrast; an unrecordedarea oriented perpendicular to the principal axis of the HOE wherein theunrecorded area comprises photopolymer material with a uniformrefractive index; and a boundary area oriented perpendicular to theprincipal axis of the HOE positioned between the recorded area and theunrecorded area wherein the boundary area includes hologram fringescomprising a photopolymer material with a second amount of refractiveindex contrast, wherein the second amount of refractive index contrastis less than the first amount of refractive index contrast, and whereinthe boundary area has a thickness measured in at least one directionperpendicular to the principal axis of the HOE less than a thickness ofthe HOE measured parallel to the principal axis of the HOE.

The thickness of the HOE as measured parallel to the principal axis ofthe HOE may be selected from a group consisting of: less than onemillimeter, less than one hundred micrometers, and less than sixmicrometers. The HOE may further include a protective layer carried bythe photopolymer layer. The HOE may be curved around a center or axis ofcurvature located on an eye-side thereof.

The HOE may comprise N layers of photopolymer, where N is an integergreater than or equal to 1, and wherein each of the N layers ofphotopolymer includes: a respective one of N recorded areas, whereineach recorded area includes a respective one of N sets of hologramfringes that define a respective one of N holograms wherein eachrespective set of hologram fringes comprise a photopolymer material witha respective one of N first amounts of refractive index contrast; arespective one of N unrecorded areas, wherein each unrecorded areacomprises a photopolymer material with a uniform refractive index; and arespective one of N boundary areas, wherein each boundary area ispositioned between each respective recorded area and each respectiveunrecorded area, wherein each boundary area includes a respective one ofN sets of secondary hologram fringes comprising a photopolymer materialwith a respective one of N second amounts of refractive index contrast,wherein each second amount of refractive index contrast is less thaneach respective first amount of refractive index contrast, and whereinthe thickness of each of the N boundary areas as measured in at leastone direction perpendicular to the principal axis of the HOE is lessthan the thickness each respective photopolymer layer as measuredparallel to the principal axis of the HOE. The recorded area may includeM multiplexed holograms, wherein M is an integer greater than or equalto 1.

The at least one hologram may comprise a reflection hologram. The atleast one hologram may comprise at least one angle-multiplexed hologram.The at least one hologram may comprise at least onewavelength-multiplexed hologram. The at least one wavelength-multiplexedhologram may comprise a red hologram, a green hologram, and a bluehologram. The at least one wavelength-multiplexed hologram may comprisean infrared hologram. The at least one hologram may comprise a hologramwith a redirection angle greater than 45 degrees. The recorded area mayhave a thickness measured in at least one direction perpendicular to theprincipal axis of the HOE less than 2 millimeters. The recorded area maycomprise a holographic incoupler.

A holographic recording medium (“HRM”) comprising a single contiguouslayer of holographic material may be summarized as including: arecording area, wherein in the recording area a holographic material ofthe HRM is photopolymerizable to a first degree; a bleached area,wherein in the bleached area the holographic material of the HRM is notphotopolymerizable; and a boundary area positioned between the recordedarea and the unrecorded area, wherein in the boundary area theholographic material of the HRM is photopolymerizable to a seconddegree, wherein the first degree to which the holographic material ofthe HRM is photopolymerizable in the recording area is higher than thesecond degree to which the holographic material of the HRM isphotopolymerizable in the boundary area and wherein the boundary areahas a thickness as measured in at least one direction perpendicular tothe principal axis of the HRM less than the thickness of the HRM asmeasured parallel to the principal axis of the HRM.

The thickness of the HRM as measured parallel to the principal axis ofthe HRM may be selected from a group consisting of: less than onemillimeter, less than one hundred micrometers, and less than sixmicrometers. The HRM may further include a protective layer carried bythe holographic material layer. The protective layer may include a firstprotective layer and a second protective layer, wherein the firstprotective layer and the second protective layer cover opposing surfacesof the photopolymer layer, and wherein at least one of the firstprotective layer and the second protective layer comprise a provisionalprotective layer. The recording area may have a thickness measured in atleast one direction perpendicular to the principal axis of the HOE lessthan 2 millimeters.

A method of fabricating a holographic recording medium (“HRM”) may besummarized as including: applying a mask to a layer of holographicmaterial, the mask comprising: at least one obstructive area wherein theat least one obstructive area is configured to shield a portion of thelayer of holographic material from light exposure; and at least onepermissive area wherein the at least one permissive area is configuredto expose a portion of the layer of holographic material to light;bleaching the masked layer of holographic material, wherein bleachingthe masked layer of holographic material includes exposing the at leastone permissive area of the mask to light; and removing the mask from thelayer of holographic material.

The layer of holographic material may include a front surface and a backsurface, and applying a mask to the layer of holographic material mayinclude: applying a front mask to the front surface of the layer ofholographic material, wherein the front mask comprises at least onepermissive area and at least one obstructive area; and applying a backmask to the back surface of the layer of holographic material, whereinthe back mask comprises a single obstructive area.

Applying a mask to a layer of holographic material may include applyinga mask to a layer of holographic material wherein the mask includes atleast one obstructive area with a shape selected from a group consistingof: a circle, an oval, a triangle, a square, a rectangle, a hexagon, andan octagon.

A method of recording a hologram may be summarized as including:mounting a layer of holographic material in an aperture-free hologramrecording assembly; applying a mask to the layer of holographicmaterial, the mask comprising: at least one obstructive area wherein theat least one obstructive area is configured to shield a portion of thelayer of holographic material from light exposure; and at least onepermissive area wherein the at least one permissive area is configuredto expose a portion of the layer of holographic material to light;illuminating the layer of holographic material with laser light, whereinilluminating the layer of holographic material with laser light includesrouting laser light from a laser light source along an aperture-freeoptical path to the layer of holographic material; removing the maskfrom the HRM; and bleaching the HRM.

Routing the laser light from the laser light source along anaperture-free optical path to the layer of holographic material mayinclude: splitting the laser light with a beamsplitter to form an objectbeam and a reference beam; collimating the object beam; routing theobject beam to illuminate the layer of holographic material; collimatingthe reference beam; and routing the reference beam to illuminate thelayer of holographic material.

The layer of holographic material may include a front surface and a backsurface, and applying a mask to the layer of holographic material mayinclude: applying a front mask to the front surface of the layer ofholographic material, wherein the front mask comprises at least onepermissive area and at least one obstructive area; and applying a backmask to the back surface of the layer of holographic material, whereinthe back mask comprises at least one permissive area and at least oneobstructive area, and wherein the back mask is positioned and orientedsuch that: each permissive area of the back mask is aligned with arespective permissive area of the front mask along the principal axis ofthe layer of holographic material; and each obstructive area of the backmask is aligned with a respective obstructive area of the front maskalong the principal axis of the layer of holographic material.

Applying a front mask to the front surface of the layer of holographicmaterial may include applying a front mask to the front surface of thelayer of holographic material wherein the front mask comprises at leastone permissive area with a shape selected from a group consisting of: acircle, an oval, a triangle, a square, a rectangle, a hexagon, and anoctagon; and applying a back mask to the back surface of the layer ofholographic material may include applying a back mask to the backsurface of the layer of holographic material wherein the back maskcomprises at least one permissive area with a shape chosen from a groupconsisting of: a circle, an oval, a triangle, a square, a rectangle, ahexagon, and an octagon.

The method may further include: pre-bleaching the layer of holographicmaterial subsequent to applying a mask to the layer of holographicmaterial, wherein applying a mask to the layer of holographic materialincludes applying a negative mask to the layer of holographic material.Illuminating the layer of holographic material with laser light mayinclude illuminating the layer of holographic material with laser lightthat comprises N different wavelengths of laser light, where N is aninteger greater than 1. Illuminating the layer of holographic materialwith laser light may include concurrently illuminating a same surface ofthe layer of holographic material with both a laser light reference beamand a laser light object beam.

Illuminating the layer of holographic material with laser light mayinclude concurrently illuminating a first surface of the layer ofholographic material with a laser light reference beam and a secondsurface of the layer of holographic material with a laser light objectbeam, wherein the second surface of the layer of holographic material islocated opposite the first surface of the layer of holographic material.Illuminating the layer of holographic material with laser light mayinclude illuminating the layer of holographic material with at least onelaser light reference beam and at least two laser light object beams.Illuminating the layer of holographic material with laser light mayinclude illuminating the layer of holographic material with laser lightgenerated by a laser light source wherein the laser light sourcecomprises an aperture. Illuminating the layer of holographic materialwith laser light may include illuminating the layer of holographicmaterial with laser light that comprises N different angles, where N isan integer greater than 1.

A method of recording a hologram may be summarized as including:mounting a layer of holographic material in an aperture-free hologramrecording assembly; applying a mask to the layer of holographicmaterial, the mask comprising: at least one obstructive area wherein theat least one obstructive area is configured to shield a portion of thelayer of holographic material from light exposure; and at least onepermissive area wherein the at least one permissive area is configuredto expose a portion of the layer of holographic material to light;generating a laser light signal with at least one laser light source;splitting the laser light signal with a beamsplitter to form an objectbeam and a reference beam; routing the object beam to illuminate thelayer of holographic material; shaping the object beam to a desiredcross-section at the layer of holographic material; routing thereference beam to illuminate the layer of holographic material; shapingthe reference beam to a desired cross-section at the layer ofholographic material; generating a pattern of optical fringes in atleast a portion of the layer of holographic material by a combination ofthe reference beam and the object beam; recording the pattern of opticalfringes as a pattern of physical fringes in at least a portion of thelayer of holographic material; removing the mask from the layer ofholographic material; and bleaching the layer of holographic material.

The layer of holographic material may include a front surface and a backsurface, and applying a mask to the layer of holographic material mayinclude: applying a front mask to the front surface of the layer ofholographic material, wherein the front mask comprises at least onepermissive area and at least one obstructive area; and applying a backmask to the back surface of the layer of holographic material, whereinthe back mask comprises at least one permissive area and at least oneobstructive area, and wherein the back mask is positioned and orientedsuch that: each permissive area of the back mask is aligned with arespective permissive area of the front mask along the principal axis ofthe layer of holographic material; and each obstructive area of the backmask is aligned with a respective obstructive area of the front maskalong the principal axis of the layer of holographic material.

Applying a front mask to the front surface of the layer of holographicmaterial may include applying a front mask to the front surface of thelayer of holographic material wherein the front mask comprises at leastone permissive area with a shape selected from a group consisting of: acircle, an oval, a triangle, a square, a rectangle, a hexagon, and anoctagon; and applying a back mask to the back surface of the layer ofholographic material may include applying a back mask to the backsurface of the layer of holographic material wherein the back maskcomprises at least one permissive area with a shape chosen from a groupconsisting of: a circle, an oval, a triangle, a square, a rectangle, ahexagon, and an octagon. The method may further include: pre-bleachingthe layer of holographic material subsequent to applying a mask to thelayer of holographic material, wherein applying a mask to the layer ofholographic material includes applying a negative mask to the layer ofholographic material.

Generating a laser light signal may include generating a laser lightsignal comprising N wavelengths of laser light, where N is an integergreater than 1, and generating a pattern of optical fringes in at leasta portion of the layer of holographic material by a combination of thereference beam and the object beam may include generating N sub-patternsof optical fringes in at least a portion of the layer of holographicmaterial by the combination of the reference beam and the object beam;and recording the pattern of optical fringes as a pattern of physicalfringes in at least a portion of the layer of holographic material mayinclude recording the N sub-patterns of optical fringes as Nsub-patterns of physical fringes in at least a portion of the layer ofholographic material.

Routing the object beam to illuminate the layer of holographic materialmay include routing the object beam to illuminate a first surface of thelayer of holographic material, and routing the reference beam toilluminate the layer of holographic material may include routing thereference beam to illuminate the first surface of the layer ofholographic material. Routing the object beam to illuminate the layer ofholographic material may include routing the object beam to illuminate afirst surface of the layer of holographic material, and routing thereference beam to illuminate the layer of holographic material mayinclude routing the reference beam to illuminate a second surface of thelayer of holographic material, wherein the first surface of the layer ofholographic material and the second surface of the layer of holographicmaterial are opposite surfaces of the layer of holographic material.

Generating a laser light signal may include generating a laser lightsignal with a laser light source wherein the laser light sourcecomprises an aperture. Generating a laser light signal may includegenerating a laser light signal comprising M angles, where M is aninteger greater than 1, and wherein generating a pattern of opticalfringes in at least a portion of the layer of holographic material by acombination of the reference beam and the object beam may includegenerating M sub-patterns of optical fringes in at least a portion ofthe layer of holographic material by the combination of the referencebeam and the object beam; and recording the pattern of optical fringesas a pattern of physical fringes in at least a portion of the layer ofholographic material may include recording the M sub-patterns of opticalfringes as N sub-patterns of physical fringes in at least a portion ofthe layer of holographic material.

A method of recording a hologram may be summarized as including:mounting a layer of holographic material in an aperture-free hologramrecording assembly; applying a mask to a layer of holographic material,the mask comprising: at least one obstructive area wherein the at leastone obstructive area is configured to shield a portion of the layer ofholographic material from light exposure; and at least one permissivearea wherein the at least one permissive area is configured to expose aportion of the layer of holographic material to light; generating alaser light signal with at least one laser light source; splitting thelaser light signal with at least one beamsplitter to form N object beamsand M reference beams, where N and M are both integers that are greaterthan or equal to 1; routing the N object beams to illuminate the layerof holographic material; shaping the N object beams to N respectivecross-sections at the layer of holographic material; routing the Mreference beams to illuminate the layer of holographic material; andshaping the M reference beams to M respective cross-sections at thelayer of holographic material; generating a pattern of optical fringesin at least a portion of the layer of holographic material by acombination of the M reference beams and the N object beams; recordingthe pattern of optical fringes as a pattern of physical fringes in atleast a portion of the layer of holographic material; removing the maskfrom the layer of holographic material; and bleaching the layer ofholographic material.

The method may further include: pre-bleaching the layer of holographicmaterial subsequent to applying a mask to the layer of holographicmaterial, wherein applying a mask to the layer of holographic materialincludes applying a negative mask to the layer of holographic material.

The layer of holographic material may comprise a front surface and aback surface, and applying a mask to the layer of holographic materialmay include: applying a front mask to the front surface of the layer ofholographic material, wherein the front mask comprises at least onepermissive area and at least one obstructive area; and applying a backmask to the back surface of the layer of holographic material, whereinthe back mask comprises at least one permissive area and at least oneobstructive area, and wherein the back mask is positioned and orientedsuch that: each permissive area of the back mask is aligned with arespective permissive area of the front mask along the principal axis ofthe layer of holographic material; and each obstructive area of the backmask is aligned with a respective obstructive area of the front maskalong the principal axis of the layer of holographic material.

Applying a front mask to the front surface of the layer of holographicmaterial may include applying a front mask to the front surface of thelayer of holographic material wherein the front mask comprises at leastone permissive area with a shape selected from a group consisting of: acircle, an oval, a triangle, a square, a rectangle, a hexagon, and anoctagon; and applying a back mask to the back surface of the layer ofholographic material may include applying a back mask to the backsurface of the layer of holographic material wherein the back maskcomprises at least one permissive area with a shape chosen from a groupconsisting of: a circle, an oval, a triangle, a square, a rectangle, ahexagon, and an octagon.

Splitting the laser light signal with at least one beamsplitter to formN object beams and M reference beams may include splitting the laserlight signal to form N object beams wherein each of the N object beamspossesses a different angle than each of the other N object beams.Splitting the laser light signal with at least one beamsplitter to formN object beams and M reference beams may include splitting the laserlight signal to form M reference beams wherein each of the M referencebeams possesses a different angle than each of the other M referencebeams.

Generating a laser light signal may include generating a laser lightsignal comprising L wavelengths of laser light, where L is an integergreater than 1, and wherein generating a pattern of optical fringes inat least a portion of the layer of holographic material by a combinationof the M reference beams and the N object beams may include generating Lsub-patterns of optical fringes in at least a portion of the layer ofholographic material for each of the combinations of the M referencebeams and the N object beams; and recording the pattern of opticalfringes as a pattern of physical fringes in at least a portion of thelayer of holographic material may include recording the L sub-patternsof optical fringes as L sub-patterns of physical fringes in at least aportion of the layer of holographic material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not necessarily drawn to scale, and some ofthese elements are arbitrarily enlarged and positioned to improvedrawing legibility. Further, the particular shapes of the elements asdrawn are not necessarily intended to convey any information regardingthe actual shape of the particular elements, and have been solelyselected for ease of recognition in the drawings.

FIG. 1 is a top-elevational view of typical hologram recording assembly100.

FIG. 2A is a cross-sectional view of HOE 200 in accordance with thepresent systems, devices, and methods.

FIG. 2B is a side elevational view of HOE 200 in accordance with thepresent systems, devices, and methods.

FIG. 3A is a cross-sectional view of HRM 300 in accordance with thepresent systems, devices, and methods.

FIG. 3B is a side elevational view of HRM 300 in accordance with thepresent systems, devices, and methods.

FIG. 4 is a flow-diagram showing a method 400 of fabricating a HRM inaccordance with the present systems, devices, and methods.

FIG. 5A is a side elevational view of masked layer of holographicmaterial 500 in accordance with the present systems, devices, andmethods.

FIG. 5B is a front elevational view of masked layer of holographicmaterial 500 in accordance with the present systems, devices, andmethods.

FIG. 6 is a top-elevational view of aperture-free hologram recordingassembly 600 in accordance with the present systems, devices, andmethods.

FIG. 7 is a flow-diagram showing a method 700 of recording a hologram inaccordance with the present systems, devices, and methods.

FIG. 8 is a flow-diagram showing a method 800 of recording a hologram inaccordance with the present systems, devices, and methods.

FIG. 9 is a flow-diagram showing a method 900 of recording a hologram inaccordance with the present systems, devices, and methods.

FIG. 10 is a top elevational view of curved HOE 1000 in accordance withthe present systems, devices, and methods.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures associated with portable electronicdevices and head-worn devices, have not been shown or described indetail to avoid unnecessarily obscuring descriptions of the embodiments.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its broadest sense, that is as meaning “and/or”unless the content clearly dictates otherwise.

The headings and Abstract of the Disclosure provided herein are forconvenience only and do not interpret the scope or meaning of theembodiments.

The various embodiments described herein provide systems, devices, andmethods for aperture-free hologram recording and are particularlywell-suited for use in holographic displays, particularly holographicdisplays used in wearable heads-up displays.

A typical hologram recording assembly records multiple unintendedholograms within and outside the desired hologram recording area inaddition to recording the intended hologram. The use of apertures in thehologram recording assembly creates unintended holograms becauseapertures diffract and reflect light and the diffracted and/or reflectedlight creates additional patterns of optical fringes. Eliminatingapertures from the hologram recording assembly eliminates the recordingof at least some of the unintended holograms, however apertures performimportant functions during hologram recording and therefore novelsystems, devices, and methods are required to record holograms in anaperture-free hologram recording assembly.

A hologram may comprise a holographic optical element HOE. In someimplementations, the HOE may be carried on or by another structure. Forinstance, one or more HOEs may be carried on or by a waveguide orlightguide structure and may serve as, for example, an in-coupler,out-coupler, or exit pupil expander for such waveguide or lightguidestructure. Thus, for the purposes of the present systems, device, andmethods, including the appended claims, the term “HOE” includes adiffractive material combined with waveguide/lightguide structures.Likewise, when the term “HOE” is used, the HOE may be carried on or byother structures or layers, or may itself carry other structures orlayers, depending on the specific implementation.

When a hologram is illuminated with light with a wavelength and anglematching the reference beam used to record the hologram, the hologramdiffracts the reference beam to create light with a wavelength and anglematching the object beam used to record the hologram (i.e. the hologramplays back the reference beam to form the object beam). The hologram maydirect the object beam in one of two directions. If the light playedback travels in the opposite direction of the reference beam (in otherwords, the playback light appears to be reflected by the hologram), thehologram is referred to as a reflection hologram. A reflection hologrammay be recorded with an object beam and a reference beam positioned onthe same side of a holographic recording medium. If the light playedback travels in the same direction as the reference beam (in otherwords, the playback light appears to have been transmitted through thehologram), the hologram is referred to as a transmission hologram. Atransmission hologram may be recorded with an object beam and areference beam positioned on opposite sides of a holographic recordingmedium.

The object beam played back by the hologram may have a different anglethan the reference beam, the difference in angle between the object beamand the reference beam played back by the hologram is the redirectionangle.

FIG. 1 is a top-elevational view of typical hologram recording assembly100. Typical hologram recording assembly 100 comprisesaperture-containing optical path 110, holographic recording medium(“HRM”) 120, beamsplitter 130, object beam 141, reference beam 142,object beam mirror 151, object beam mirror 152, reference beam mirror153, object beam aperture assembly 154, reference beam aperture assembly155, collimating aperture assembly 160, laser light source 170 andbaffle 180. Each of object beam aperture assembly 154, reference beamaperture assembly 155, and collimating aperture assembly 160 comprise arespective aperture, focusing lens, and collimating lens.

Recording a hologram requires the generation of a pattern of opticalfringes with laser light; the pattern of optical fringes is thenrecorded as a pattern of physical fringes in a HRM. The laser light maybe generated by a laser light source; the laser light may then bemanipulated to generate a pattern of optical fringes. Non-exclusiveexamples of laser light manipulations include focusing the laser lightwith a lens, reflecting the laser light with a mirror, and blocking aportion of the laser light with an aperture. Laser light source 170generates a laser light signal. Beamsplitter 130 splits the laser lightsignal into object beam 141 and reference beam 142. Object beam mirror151 and object beam mirror 152 route object beam 141 to HRM 120.Reference beam mirror 153 routes reference beam 142 to HRM 120. Thecombination of object beam 141 and reference beam 142 at HRM 120 createsa pattern of optical fringes, the pattern of optical fringes is recordedas a pattern of physical fringes in HRM 120.

The pattern of optical fringes is recorded as a pattern of physicalfringes via a reaction between the HRM and the laser light comprisingthe pattern of optical fringes. The reaction between the HRM and thelaser light may be physical; non-exclusive examples of physicalreactions include melting, ablation, and light-induced changes inrefractive index. The reaction between the HRM and may be chemical, anon-exclusive example of a chemical reaction is photopolymerization.

In a photopolymerization, a photoinitiator absorbs light and producesactive centers. Non-exclusive examples of active centers include anions,cations, and free radicals. The active centers convert a monomer to apolymer until all of the available monomer has reacted or until theactive centers are destroyed by at least one quenching reaction. Aphotopolymerizable HRM comprises a photoinitiator, a monomer, and amatrix polymer. The monomer may be converted to photopolymer by exposingthe HRM to light. The matrix polymer has a first refractive index andthe photopolymer has a second refractive index; the first refractiveindex may be higher than the second refractive index and the firstrefractive index may be lower than the second refractive index. Thepattern of optical fringes is recorded in the photopolymerizable HRM asa pattern of photopolymer, where the pattern of photopolymer comprises apattern of higher or lower refractive index.

A typical HRM is larger than the desired hologram to allow variation inthe size and position of the recorded hologram. The position of thepattern of optical fringes determines the position of the hologram andthe position of the pattern of optical fringes may be controlled byrouting the laser light with mirrors. The spot size of the laser lightat the HRM typically determines the size of the recorded hologram andthe spot size of the laser light may be controlled by blocking a portionof the laser light with an aperture. Object beam aperture assembly 154,reference beam aperture assembly 155, and collimating aperture assembly160 each comprise an aperture that blocks a portion of the laser lightto control the size of the hologram recorded in HRM 120.

Blocking a portion of the laser light with an aperture isdisadvantageous since the laser light will diffract as it passes thesharp edge of the movable blade of the aperture. Diffracting the laserlight may produce an Airy disk around the beam of laser light, where anAiry disk comprises a series of rings of laser light surrounding theprimary beam of laser light. The distance between the primary beam oflaser light and the rings of the Airy disk is determined by the distanceover which the diffracting laser light can propagate. No observable Airydisk is formed immediately adjacent to the aperture, however the Airydisk becomes visible, and the distance between the primary beam of laserlight and the rings of the Airy disk increases, as the distance betweenthe aperture and HRM 120 increases.

Multiple apertures along aperture-containing optical path 110 causesuccessive diffractions of the laser light, resulting in complexpatterns of optical fringes within and outside the intended hologramrecording area in HRM 120 in addition to the intended pattern of opticalfringes created by the combination of object beam 141 and reference beam142.

Apertures are typically constructed of light-absorbing materials toreduce the amount of stray light generated by blocking a portion of thelaser light signal. The light absorbing-materials used in apertureconstruction are not perfectly efficient, therefore blocking a portionof laser light with an aperture will generate some stray light. Straylight can reach the HRM and create an unintended pattern of opticalfringes that may be recorded as an unintended pattern of physicalfringes in HRM 120. Baffle 180 is typically included in conventionalhologram recording assembly 100 in an attempt to prevent stray lightfrom reaching HRM 120. Baffle 180 is typically constructed oflight-absorbing materials that are not perfectly efficient and mayreflect stray light, therefore the inclusion of baffle 180 in typicalhologram recording assembly 100 does not necessarily reduce the amountof stray light that reaches HRM 120 and may in fact increase the amountof stray light reaching HRM 120.

A person of skill in the art would appreciate that, in the variousembodiments described herein, photopolymer is used as an exemplaryholographic material. Unless the specific context requires otherwise,the present systems, devices, and methods can be employed withholographic materials other than photopolymer e.g. photographicemulsion, dichromated gelatin, photothermoplastics, andphotorefractives, and references to photopolymer should generally beconstrued to encompass any holographic material.

FIG. 2A is a cross-sectional view of holographic optical element (“HOE”)200 in accordance with the present systems, devices, and methods. HOE200 comprises a single contiguous layer of photopolymer material. HOE200 includes recorded area 210, unrecorded area 220, and boundary area230. Throughout this specification and the appended claims, the term“layer” generally refers to a thickness of some material that providesand/or is spread over a surface, such as a stratum or a coating on asurface. A layer may include or cover a single side or face of astructure, such as a dielectric layer in a printed circuit board or alayer of cheese on a pizza, or a layer may include or cover multiplesides or faces of a three-dimensional structure, such as a layer ofclothing or a layer of planet Earth (e.g., the crust, mantle, etc.). Aperson of skill in the art will appreciate that the material of onelayer may form the substrate of another layer.

Recorded area 210 includes hologram fringes that define at least onehologram. Recorded area 210 is oriented perpendicular to the principalaxis of HOE 200. Throughout this specification and the appended claims,the term “principal axis” generally refers to the line parallel to thesmallest dimension of a holographic optical element or a holographicrecording medium. An exemplary HOE has a thickness in a first dimensionless than two millimeters, and a thickness in a second dimension and athickness in a third dimension greater than one centimeter. Theprinciple axis of the exemplary HOE would therefore be parallel to thefirst dimension and perpendicular to the second and third dimensions.

The hologram fringes of recorded area 210 comprise a photopolymermaterial with a first amount of refractive index contrast. The firstamount of refractive index contrast may be greater than 0.005, greaterthan 0.016, or greater than 0.06. A refractive index contrast greaterthan 0.005 is advantageous since holograms with a refractive indexcontrast below 0.005 typically do not possess sufficiently highdiffraction efficiencies for photopolymer of a typical thickness. Arefractive index contrast greater than 0.016 and/or greater than 0.06may be advantageous as a higher refractive index contrast typicallycauses a hologram to have a higher efficiency, however a person of skillin the art will appreciate that an excessively high refractive indexcontrast may cause the hologram to be overmodulated, reducing theefficiency of the hologram. Unrecorded area 220 comprises photopolymermaterial with a uniform refractive index. Photopolymer material with auniform refractive index contains no hologram fringes. The absence ofhologram fringes in unrecorded area 200 includes the absence of hologramfringes from unwanted secondary holograms. Unrecorded area 220 isoriented perpendicular to the principal axis of HOE 200.

Boundary area 230 includes hologram fringes comprising a photopolymermaterial with a second amount of refractive index contrast wherein thesecond amount of refractive index contrast is less than the first amountof refractive index contrast. The hologram fringes of boundary area 230comprise a hologram with a lower diffraction efficiency than thehologram located in recorded area 210 due to the lower refractive indexcontrast of the hologram fringes of boundary area 230.

Boundary area 230 is oriented perpendicular to the principal axis of HOE200. Boundary area 230 has a thickness as measured in at least onedirection perpendicular to the principal axis of HOE 200 less than athickness of HOE 200 measured parallel to the principal axis of HOE 200.The thickness of boundary area 230 as measured perpendicular to theprincipal axis of HOE 200 may be less than one millimeter, less than onehundred micrometers, or less than six micrometers. Boundary area 230 ispositioned between recorded area 210 and unrecorded area 220. Thelimited thickness and reduced diffraction efficiency of boundary area230 may be created by recording HOE 200 in an aperture-free hologramrecording assembly.

The at least one hologram defined by the hologram fringes in recordedarea 210 may comprise a wavelength-multiplexed hologram. A wavelengthmultiplexed hologram comprises at least two wavelength-specificholograms, wherein each wavelength-specific hologram has a respectiveplayback wavelength; each wavelength-specific hologram may have arespective incident playback angle and a respective redirection angle. Awavelength multiplexed hologram may include a red hologram, a greenhologram, and a blue hologram, which advantageously allows the hologramto be used in a full-color display (as a holographic combiner or as aholographic incoupler/outcoupler). A wavelength multiplexed hologram mayinclude an infrared hologram, which advantageously may be employed ineye tracking applications.

The at least one hologram defined by the hologram fringes in recordedarea 210 may possess a redirection angle greater than 30 degrees,greater than 45 degrees, or greater than 60 degrees. A high redirectionangle is advantageous for HOEs employed as incouplers and/or outcouplersin light guides, since a higher redirection angle increases theresolution of light guide based displays.

Recorded area 210 may possess a thickness measured in at least onedirection perpendicular to the principal axis of HOE 200 less than 2millimeters; a HOE with a smaller thickness in at least one directionperpendicular to the principal axis of the HOE is advantageous for useas a holographic incoupler for a light guide based display, as a smallerincoupler increases the resolution of said display.

FIG. 2B is a side elevational view of HOE 200 in accordance with thepresent systems, devices, and methods. HOE 200 may comprise a protectivelayer 240 carried by the single contiguous layer of photopolymer of HOE200. Non-exclusive examples of protective layer materials includeacrylic, polystyrene, and polycarbonate. Protective layer 240 isphysically coupled to HOE 200. The protection provided by protectivelayer 240 includes protection from scratches, tears, and water damage.

HOE 200 may be curved around a center or axis of curvature located on aneye-side of HOE 200. A curved HOE may be a spherically curved HOE; aspherically curved HOE is curved around a center of curvature. A curvedHOE may be a cylindrically curved HOE; a cylindrically curved HOE iscurved around an axis of curvature. The center or axis of curvature, asappropriate, of HOE 200 may be located on the eye-side of HOE 200 at adistance of between 1 and 10 centimeters, between 10 and 50 cm, orbetween 50 and 100 cm from HOE 200.

Throughout this specification and the appended claims, the term“eye-side” refers to the side of the object that, when employed in adevice worn by a user, faces towards the eye of the user, while the term“world-side” refers to the side of the eyeglass lens that, when employedin a device worn by a user, faces away from the eye of the user andtowards the outside world.

A curved HOE may be more easily incorporated into curved lenses for useas a transparent combiner in a wearable heads-up display (“WHUD”);curved lenses have greater aesthetic appeal than planar lenses. Recordedarea 210 may include M multiplexed holograms, where M is an integergreater or equal to 1. The M multiplexed holograms may bewavelength-multiplexed holograms, angle multiplexed holograms, or anycombination thereof.

Recorded area 210, unrecorded area 220, and boundary area 230 comprise asingle layer of photopolymer. HOE 200 may comprise N layers ofphotopolymer, where N is an integer greater than or equal to 1. Each ofthe N layers of photopolymer include a respective one of N recordedareas 210, a respective one of N unrecorded areas 220, and a respectiveone of N boundary areas 230.

Each of the N recorded areas 210 includes a respective one of N sets ofhologram fringes that define at least one hologram. Each set of hologramfringes comprise a photopolymer material with a respective one of Nfirst amounts of refractive index contrast. Each of the N unrecordedareas 220 comprises a photopolymer with a uniform refractive index.

Each of the N boundary areas 230 is positioned between each respectiverecorded area and each respective unrecorded area. Each boundary area230 includes a respective one of N sets of secondary hologram fringes;each set of secondary hologram fringes comprises a photopolymer materialwith a respective one of N second amounts of refractive index contrast.Each of the N second amounts of refractive index contrast is less thaneach respective first amount of refractive index contrast. The thicknessof each of the N boundary areas, as measured in at least one directionperpendicular to the principal axis of the HOE, is less than thethickness of each one of the N boundary areas as measured parallel tothe principal axis of the HOE.

FIG. 3A is a cross-sectional view of holographic recording medium(“HRM”) 300 in accordance with the present systems, devices, andmethods. HRM 300 comprises a single contiguous layer of holographicmaterial. HRM 300 comprises recording area 310, bleached area 320 andboundary area 330. In recording area 310 a holographic material of HRM300 is photopolymerizable to a first degree. In bleached area 320 theholographic material of HRM 300 is not photopolymerizable.

The degree to which a holographic material is photopolymerizable isdetermined by the amount of photoinitiator and the amount of monomerpresent in the holographic material. A holographic material isphotopolymerizable if the holographic material contains bothphotoinitiator and monomer. A holographic material is notphotopolymerizable if the holographic material lacks sufficientquantities of either photoinitiator or monomer to produce photopolymerupon exposure to light. A holographic material with a greater amount ofphotoinitiator and a greater amount of monomer is morephotopolymerizable than a holographic material with a lesser amount ofphotoinitiator and a lesser amount of monomer. Exposing a holographicmaterial to light causes the holographic material to become lessphotopolymerizable by consuming both photoinitiator and monomer.

Boundary area 330 is positioned between recorded area 310 and bleachedarea 320. In boundary area 320 the holographic material of HRM 300 isphotopolymerizable to a second degree. The first degree to which theholographic material of HRM 300 is photopolymerizable in recording area310 is higher than the second degree to which the holographic materialof HRM 300 is polymerizable in boundary area 330. Boundary area 330 hasa thickness as measured in at least one direction perpendicular to theprincipal axis of HRM 300 less than a thickness of HRM 300 measuredparallel to the principal axis of HRM 300. The thickness of boundaryarea 330 as measured perpendicular to the principal axis of HRM 300 maybe less than one millimeter, less than one hundred micrometers, lessthan six micrometers.

A hologram may be recorded in HRM 300. The size of the hologram recordedin HRM 300 may not exceed the area of recording area 310 and boundaryarea 300 combined; the size of the hologram will not significantlyexceed the area of recording area 310 due to the limited dimensions ofboundary area 330 as described above. The size limits imposed onhologram recording by the size of recording area 310 are advantageous,since this allows the size of the hologram to be determined by thephysical dimensions of recording area 310 rather than the spot sizes ofbeams of laser light used to record holograms. The spot size of a beamof laser light is typically controlled by an aperture. Eliminating theneed to control spot sizes when recording a hologram in HRM 300therefore eliminates the need for apertures when recording a hologram inHRM 300.

Recording area 310 may possess a thickness measured in at least onedirection perpendicular to the principal axis of HRM 300 less than 2millimeters; a recording area with a smaller thickness in at least onedirection perpendicular to the principal axis of the HOE is advantageousfor use as a recording material for holographic incouplers for a lightguide based display, as a smaller incoupler increases the resolution ofsaid display.

FIG. 3B is a side elevational view of HRM 300 in accordance with thepresent systems, devices, and methods. HRM 300 may comprise a protectivelayer 340 carried by the single contiguous layer of holographic materialof HRM 300. Protective layer 340 may comprise first protective layer 360and second protective layer 370. First protective layer 360 and secondprotective layer 370 cover opposing surfaces of the single contiguouslayer of holographic material of HRM 300. At least one of firstprotective layer 360 and second protective layer 370 may comprise aprovisional protective layer. A provisional protective layer may bephysically de-coupled from HRM 300 without causing damage to HRM 300. Aprovisional protective layer is advantageous as it protects HRM 300 fromdamage during processing and may then be removed prior to any subsequentprocesses that are incompatible with the protective layer.

FIG. 4 is a flow-diagram showing a method 400 of fabricating a HRM inaccordance with the present systems, devices, and methods. Method 400includes three acts 401, 402, and 403 though those of skill in the artwill appreciate that in alternative embodiments certain acts may beomitted and/or additional acts may be added. Those of skill in the artwill also appreciate that the illustrated order of the acts is shown forexemplary purposes only and may change in alternative embodiments.

As an illustrative example of the physical elements of method 400,analogous structures from FIG. 5 are called out in parenthesesthroughout the description of acts 401, 402, and 403.

At 401, a mask (520) is applied to a layer of holographic material(510). The mask (520) is a layer of material that is provisionallyphysically coupled to the layer of holographic material (510); the mask(520) may be applied to the layer of holographic material (510) and themask (520) may also be removed from the layer of holographic material(510). The mask (520) comprises at least one permissive area (531) andat least one obstructive area (532). The at least one permissive area(531) allows a subsequent treatment to occur in the area of the layer ofholographic material covered by the at least one permissive area (531).The at least one obstructive area (532) prevents a subsequent treatmentfrom occurring in the area of the layer of holographic material coveredby the at least one obstructive area (532). Non-exclusive examples oftreatments include bleaching, etching, and hardening.

Permissive areas (531) may be non-contiguous. Permissive areas (531) mayhave shapes and features with an upper size limit determined by the sizeof the mask (520) since a given feature must be able to fit entirelywithin the mask (520). Permissive areas (531) may have shapes andfeatures with a lower size limit determined by the resolution limit ofthe mask fabrication method. Masks may be produced via photolithographictechniques which can produce features with a resolution limit of 50nanometers. Obstructive areas (532) may be non-contiguous. Obstructiveareas (532) may have shapes and features with an upper size limitdetermined by the size of the mask (520) and a lower size limitdetermined by the resolution limit of the mask fabrication method. Theobstructive area (532) may have a shape selected from a group consistingof: a circle, an oval, a triangle, a square, a rectangle, a hexagon, andan octagon.

The layer of holographic material (510) may comprise a front surface(511) and a back surface (512). Applying a mask (520) to a layer ofholographic material (510) may include applying a front mask (521) tothe front surface of the layer of holographic material (511). The frontmask (521) comprises at least one permissive area (531) and at least oneobstructive area (532). Applying a mask (520) to a layer of holographicmaterial (510) may include applying a back mask (522) to the backsurface of the layer of holographic material (512). The back mask (522)comprises a single obstructive area.

At 402, the masked layer of holographic material (500) is bleached.Bleaching includes exposing the masked layer of holographic material(500) to a bleaching agent. Non-exclusive examples of a bleaching agentsinclude acids, peroxides, hypochlorites, and light. Photobleachingincludes exposing a masked layer of holographic material (500) to light.The light used for photobleaching may be incoherent. The light used forphotobleaching may be polychromatic, wherein at least a portion of thelight which is used for photobleaching may be absorbed by thephotoinitiator or the monomer.

Photobleaching the masked layer of holographic material (500) convertsat least a portion of the layer of holographic material (510) from aphotopolymerizable material to a material that is notphotopolymerizable. During photobleaching, the permissive areas (531) ofthe mask (520) are transparent to at least one wavelength of the lightused for photobleaching. During photobleaching, the obstructive areas(532) of the mask (520) are opaque to all wavelengths of the light usedfor photobleaching that may be absorbed by the photoinitiator or themonomer.

At 403, the mask is removed from the layer of holographic material(510).

FIG. 5A is a side elevational view of masked layer of holographicmaterial 500 in accordance with the present systems, devices, andmethods. Masked layer of holographic material 500 includes layer ofholographic material 510 and mask 520. Layer of holographic material 510includes front surface of the layer of holographic material 511 and backsurface of the layer of holographic material 512. Mask 520 comprisesfront mask 521 and back mask 522.

FIG. 5B is a front elevational view of masked layer of holographicmaterial 500 in accordance with the present systems, devices, andmethods. Only front mask 521 is visible in FIG. 5B due to the frontelevational view; layer of holographic material 510 and back mask 522are obscured from view by front mask 521 due to the viewing angle inFIG. 5B. Front mask 521 comprises at least one permissive area 531 andat least one obstructive area 532. Back mask may be substantivelysimilar to front mask 531. Back mask 522 comprises at least oneobstructive area 532 and may comprise at least one permissive area 531.

FIG. 6 is a top-elevational view of aperture-free hologram recordingassembly 600 in accordance with the present systems, devices, andmethods. Aperture-free hologram recording assembly 600 comprises:aperture-free optical path 610, HRM 620, beamsplitter 630, object beam641, reference beam 642, object beam mirror 651, object beam mirror 652,reference beam mirror 653, object beam lens 654, reference beam lens655, collimating lens 660 and laser light source 670.

Laser light source 670 generates a beam of laser light. The beam oflaser light travels along aperture-free optical path 610. Aperture-freeoptical path 610 includes HRM 620, beamsplitter 630, object beam mirror651, object beam mirror 652, reference beam mirror 653, object beam lens654, reference beam lens 655, and collimating lens 660. Aperture-freeoptical path 610 does not include laser light source 670. Collimatinglens 660 collimates the beam of laser light after the beam of laserlight exits laser light source 670. HRM 620 may comprise a layer ofholographic material. HRM 620 may comprise a masked layer of holographicmaterial substantively similar to masked layer of holographic material500.

Beamsplitter 630 splits the beam of laser light to form object beam 641and reference beam 642. Object beam mirror 651 and object beam mirror652 route object beam 641 to illuminate HRM 620. Object beam lens 654shapes object beam 641 to a desired cross-section at HRM 620. Referencebeam mirror 653 routes reference beam 642 to illuminate HRM 620.Reference beam lens 655 shapes reference beam 642 to a desiredcross-section at HRM 620.

Aperture-free hologram recording assembly 600 may record a hologram inHRM 620 by illuminating HRM 620 with object beam 641 and reference beam642. HRM 620 may be substantively similar to HRM 300 (FIG. 3). The useof a HRM substantively similar to HRM 300 is advantageous since therecorded hologram will not be significantly larger than recording area310 (FIG. 3) regardless of the spot size of the object beam or the spotsize of the reference beam; a hologram cannot be recorded in bleachedarea 320 (FIG. 3) and boundary area 330 (FIG. 3) is typically smallrelative to the size of recording area 310. The insensitivity of HRM tolarge spot sizes allows for larger tolerances in spot size, reducing theneed for careful positioning of object beam lens 654 and reference beamlens 655.

FIG. 7 is a flow-diagram showing a method 700 of recording a hologram inaccordance with the present systems, devices, and methods. Method 700includes five acts 701, 702, 703, 704, and 705 though those of skill inthe art will appreciate that in alternative embodiments certain acts maybe omitted and/or additional acts may be added. Those of skill in theart will also appreciate that the illustrated order of the acts is shownfor exemplary purposes only and may change in alternative embodiments.

As an illustrative example of the physical elements of method 700,analogous structures from FIG. 5 and FIG. 6 are called out inparentheses throughout the description of acts 701, 702, 703, 704, and705.

At 701, a layer of holographic material (510, 620) is mounted in anaperture-free recording assembly (600). The aperture-free hologramrecording assembly (600) comprises a laser light source (670) and anaperture-free optical path (610). The laser light source (670) does notrequire an aperture in order to generate a laser light signal, the beamdiameter of the laser light signal generated by the laser light source(670) may be controlled via controlling the geometry of a resonantchamber of the laser light source (670). A person of skill in the artwill appreciate that controlling the geometry of the resonant chamber ofthe laser light source controls where and how photons of light arecreated rather than blocking light in a manner consistent with anaperture. The laser light source may comprise at least one aperture. Theaperture-free optical path (610) may comprise beam-routing mirrors (651,652, 653), and the aperture-free optical path (610) may comprisebeam-shaping lenses (654, 655), however the aperture-free optical path(610) does not comprise any apertures. The aperture-free optical path(610) does not include the laser light source (670).

At 702, a mask (520) is applied to the layer of holographic material(510, 620). The mask (520) comprises at least one obstructive area (532)wherein the at least one obstructive area (532) is configured to shielda portion of the layer of holographic material from light exposure. Themask (520) comprises at least one permissive area (531) wherein the atleast one permissive area (531) is configured to expose a portion of thelayer of holographic material to light.

The layer of holographic material (510, 620) may comprise a frontsurface (511) and a back surface (512). Applying a mask (520) to thelayer of holographic material (510, 620) may include applying a frontmask (521) to the front surface of the layer of holographic material(511). The front mask comprises at least one permissive area (531) andat least one obstructive area (532). Applying a mask (520) to the layerof holographic material (510, 620) may include applying a back mask(522) to the back surface of the layer of holographic material (512).The back mask comprises at least one permissive area (531) and at leastone obstructive area (532). Each permissive area (531) of the back mask(522) is aligned with a respective permissive area (521) of the frontmask (521) along the principal axis of the layer of holographicmaterial. Each obstructive area (532) of the back mask (522) is alignedwith a respective obstructive area (532) of the front mask along theprincipal axis of the layer of holographic material.

The front mask (521) may comprise at least one permissive area with ashape selected from a group consisting of: a circle, an oval, atriangle, a square, a rectangle, a hexagon, and an octagon. The backmask (522) may comprise at least one permissive area with a shapeselected from a group consisting of: a circle, an oval, a triangle, asquare, a rectangle, a hexagon, and an octagon.

At 703, the layer of holographic material (510, 620) is illuminated withlaser light. The layer of holographic material (510, 620) may comprise amasked layer of holographic material (500). Illuminating the layer ofholographic material (510, 620) with laser light includes routing laserlight from a laser light source (670) along an aperture-free opticalpath (610) to the layer of holographic material (510, 620). Illuminatingthe layer of holographic material (510, 620) with laser light mayinclude illuminating the layer of holographic material (510, 620) withlaser light generated by a laser light source (670) wherein the laserlight source (670) comprises an aperture. An aperture within the laserlight source (670) may create an undesirable diffraction pattern,however the distance between the laser light source (670) and the layerof holographic material (510, 620) may be large enough to ensure thatthe undesirable diffraction pattern falls on a portion of the layer ofholographic material that is covered by the obstructive area (532) ofthe mask (520) covering the layer of holographic material (510, 620).

Illuminating the layer of holographic material (510, 620) with laserlight records a hologram in the layer of holographic material (510,620). If the layer of holographic material (510, 620) comprises a maskedlayer of holographic material (500), the hologram is not recorded in anyarea of the layer of holographic material (510, 620) covered by the atleast one obstructive area (532) because the obstructive area (532)shields the layer of holographic material (510, 620) from the laserlight. If the layer of holographic material (510, 620) comprises amasked layer of holographic material (500), the hologram is recorded inthe area of the layer of holographic material (510, 620) covered by theat least one permissive area (531) because the at least one permissivearea (531) allows the layer of holographic material (510, 620) to beexposed to the laser light. If the layer of holographic material (510,620) comprises a masked layer of holographic material (500), the mask(520) limits the size and position of a recorded hologram to the area ofthe layer of holographic material (510, 620) covered by the at least onepermissive area (531) of the mask, therefore the mask (520) eliminatesthe need for apertures in the aperture-free hologram recording assembly(600).

A person of skill in the art will appreciate that a mask (520) and anaperture both block at least a portion of laser light from reaching thelayer of holographic material (510, 620) and simultaneously comprise asharp edge that diffracts laser light. A mask (520) is placed in directphysical contact with the layer of holographic material (510, 620),therefore any diffraction caused by the mask (520) can only propagatethrough a distance equal to the thickness of the layer of holographicmaterial (510, 620) and any included protective materials. The thicknessof the layer of holographic material (510, 620) is typically less thantwo millimeters and no significant diffraction can occur with such ashort propagation distance. An aperture is typically positioned at adistance of more than ten centimeters from the layer of holographicmaterial (510, 620), since an aperture is designed to allow additionalbeam manipulation further along the optical path of the beam.Significant diffraction propagation can occur over a distance of morethan ten centimeters leading to the recording of unintentional secondaryholograms in the layer of holographic material (510, 620).

Routing the laser light from the laser light source along anaperture-free optical path to the layer of holographic material mayinclude splitting the laser light with a beamsplitter to form a laserlight object beam and a laser light reference beam. Routing the laserlight from the laser light source along an aperture-free optical path tothe layer of holographic material may include collimating the laserlight object beam, collimating the laser light reference beam, routingthe laser light object beam to illuminate the layer of holographicmaterial (500), and routing the laser light reference beam to illuminatethe layer of holographic material (500).

Collimating the laser light object beam includes collimating the laserlight object beam with a collimating lens. Collimating the laser lightreference beam includes collimating the laser light reference beam witha collimating lens. Routing the laser light object beam to illuminatethe layer of holographic material (500) includes routing the laser lightobject beam with an object beam mirror (653). Routing the laser lightreference beam to illuminate the layer of holographic material (500)includes routing the laser light reference beam with a reference beammirror (651, 652)

Illuminating the layer of holographic material (510, 620) with laserlight may include illuminating the layer of holographic material (510,620) with laser light that comprises N wavelengths of laser light, whereN is an integer greater than 1. Illuminating the layer of holographicmaterial (510, 620) with laser light comprising N different wavelengthsof laser light records a wavelength-multiplexed hologram in the layer ofholographic material (510, 620). A wavelength-multiplexed hologram isadvantageous because wavelength-multiplexed holograms can producefull-color displays when used in holographic display applications.

Illuminating the layer of holographic material (510, 620) with laserlight may include illuminating the layer of holographic material (510,620) with laser light that comprises N angles, where N is an integergreater than 1. Illuminating the layer of holographic material (510,620) with laser light comprising N different angles records anangle-multiplexed hologram in the layer of holographic material (510,620). An angle-multiplexed hologram is advantageous becauseangle-multiplexed holograms possess an effectively increased angularbandwidth that increases the field of view of displays employing theangle-multiplexed hologram. Illuminating the layer of holographicmaterial (510, 620) with laser light may include concurrentlyilluminating a same surface of the layer of holographic material (510,620) with both a laser light reference beam (642) and a laser lightobject beam (641). Illuminating the layer of holographic material withlaser light concurrently with a laser light reference beam (642) and alaser light object beam (641) on a same surface records a transmissionhologram in the layer of holographic material (510, 620).

Illuminating the layer of holographic material (510, 620) with laserlight may include concurrently illuminating a first surface of the layerof holographic material of the layer of holographic material (510, 620)with a laser light reference beam (642) and a second surface of thelayer of holographic material with a laser light object beam (641). Thesecond surface of the layer of holographic material (510, 620) islocated opposite the first surface of the layer of holographic material(510, 620). Illuminating the layer of holographic material with laserlight concurrently with a laser light reference beam (642) and a laserlight object beam (641) on opposite surfaces records a reflectionhologram in the layer of holographic material (510, 620).

Illuminating the layer of holographic material (510, 620) with laserlight may include illuminating the layer of holographic material with atleast one laser light reference beam and at least two laser light objectbeams. Each of the at least two laser light object beams may illuminatethe layer of holographic material (510, 620) with a respective one of θangles, where θ is an integer greater than 1.

At 704, the mask (520) is removed from the layer of holographic material(510, 620). Removal of the mask (520) allows the areas of the layer ofholographic material (510, 620) that were previously covered by theobstructive areas (532) of the mask (520) to subsequently be exposed tolight.

At 705, the layer of holographic material (510, 620) is bleached.Bleaching the layer of holographic material (510, 620) may includephotobleaching. Photobleaching the layer of holographic material (510,620) includes exposing the layer of holographic material to light. Thelight used for photobleaching may be incoherent light. The light usedfor photobleaching may be polychromatic, wherein at least a portion ofthe light which is used for photobleaching may be absorbed by thephotoinitiator or the monomer. Photobleaching the layer of holographicmaterial (510, 620) converts at least a portion of the layer ofholographic material (510, 620) from a photopolymerizable material to amaterial that is not photopolymerizable.

Method 700 may further comprise pre-bleaching the layer of holographicmaterial (510, 620). Pre-bleaching the layer of holographic material(510, 620) includes converting a portion of the layer of holographicmaterial (510, 620) from a material that is photopolymerizable to amaterial that is not photopolymerizable. Pre-bleaching the layer ofholographic material (510, 620) may include photo-bleaching the layer ofholographic material (510, 620).

Pre-bleaching the layer of holographic material (510, 620) occurssubsequent to applying a mask (520) to the layer of holographic material(510, 620). Pre-bleaching the layer of holographic material (510, 620)occurs prior to removing the mask (520) from the layer of holographicmaterial (510, 620). If the layer of holographic material (510, 620) ispre-bleached, applying a mask (520) to the layer of holographic material(510, 620) includes applying a negative mask to the layer of holographicmaterial (510, 620). A negative mask comprises a mask (520) wherein theobstructive areas (532) cover the portion of the layer of holographicmaterial (510, 620) that, subsequent to pre-bleaching, comprises amaterial that is photopolymerizable. A negative mask comprises a mask(520) wherein the at least one permissive area (531) covers the portionof the layer of holographic material (510, 620) that, subsequent topre-bleaching, comprises a material that is not photopolymerizable. Themaximum size of a hologram that may be recorded in the layer ofholographic material (510, 620) is equal to the size of the areas of thelayer of holographic material (510, 620) that were covered by the atleast one obstructive area (532) of the negative mask. The limits on thesize of the recordable hologram imposed by pre-bleaching the layer ofholographic material (510, 620) with a negative mask eliminate the needfor apertures in the aperture-free hologram recording assembly (600).

FIG. 8 is a flow-diagram showing a method 800 of recording a hologram inaccordance with the present systems, devices, and methods. Method 800includes twelve acts 801, 802, 803, 804, 805, 806, 807, 808, 809, 810,811, and 812 though those of skill in the art will appreciate that inalternative embodiments certain acts may be omitted and/or additionalacts may be added. Those of skill in the art will also appreciate thatthe illustrated order of the acts is shown for exemplary purposes onlyand may change in alternative embodiments.

As an illustrative example of the physical elements of method 700,analogous structures from FIG. 5 and FIG. 6 are called out inparentheses throughout the description of acts 801, 802, 803, 804, 805,806, 807, 808, 809, 810, 811, and 812.

At 801, a layer of holographic material (510, 620) is mounted in anaperture-free recording assembly (600). The aperture-free hologramrecording assembly (600) comprises a laser light source (670) and anaperture-free optical path (610). The laser light source (670) does notrequire an aperture in order to generate a laser light signal, the beamdiameter of the laser light signal may be controlled via careful designof a resonant chamber of the laser light source (670). The laser lightsource may comprise at least one aperture. The aperture-free opticalpath (610) may comprise beam-routing mirrors (651, 652, 653), and theaperture-free optical path (610) may comprise beam-shaping lenses (654,655), however the aperture-free optical path (610) does not comprise anyapertures. The aperture-free optical path (610) does not include thelaser light source (670).

At 802, a mask (520) is applied to the layer of holographic material(510, 620). The mask (520) comprises at least one obstructive areawherein the at least one obstructive area is configured to shield aportion of the layer of holographic material from light exposure. Themask (520) comprises at least one permissive area wherein the at leastone permissive area is configured to expose a portion of the layer ofholographic material to light. The mask (520) eliminates the need forapertures in the aperture-free hologram recording assembly (600).

The layer of holographic material (510, 620) may comprise a frontsurface (511) and a back surface (512). Applying a mask (520) to thelayer of holographic material (510, 620) may include applying a frontmask (521) to the front surface of the layer of holographic material(511). The front mask comprises at least one permissive area (531) andat least one obstructive area (532). Applying a mask (520) to the layerof holographic material (510, 620) may include applying a back mask(522) to the back surface of the layer of holographic material (512).The back mask comprises at least one permissive area (531) and at leastone obstructive area (532). Each permissive area (531) of the back mask(522) is aligned with a respective permissive area (521) of the frontmask (521) along the principal axis of the layer of holographicmaterial. Each obstructive area (532) of the back mask (522) is alignedwith a respective obstructive area (532) of the front mask along theprincipal axis of the layer of holographic material.

The front mask (521) may comprise at least one permissive area with ashape selected from a group consisting of: a circle, an oval, atriangle, a square, a rectangle, a hexagon, and an octagon. The backmask (522) may comprise at least one permissive area with a shapeselected from a group consisting of: a circle, an oval, a triangle, asquare, a rectangle, a hexagon, and an octagon.

At 803, a laser light signal is generated with at least one laser lightsource (670). Generating a laser light signal may include generating alaser light signal comprising N wavelengths of laser light, where N isan integer greater than 1. Generating a laser light signal may includegenerating a laser light signal with a laser light source (670), whereinthe laser light source (670) comprises an aperture.

At 804, the laser light signal is split with a beamsplitter (630) toform an object beam (641) and a reference beam (642). Non-exclusiveexamples of beamsplitters include a beamsplitter cube, a Wollastonprism, and a semi-silvered mirror.

At 805, the object beam (641) is routed to illuminate the layer ofholographic material (500). Non-exclusive examples of object beamrouting components include a mirror (651, 652), a prism, split wedges,and an optical fiber. Routing the object beam to illuminate the layer ofholographic material (510, 620) may include routing the object beam toilluminate a first surface of the layer of holographic material (510,620).

At 806, the object beam (641) is shaped to a desired cross-section atthe layer of holographic material (500). Non-exclusive examples ofobject beam shaping components include a lens (654) and a diffractiveoptical element.

At 807, the reference beam (642) is routed to illuminate the layer ofholographic material (500). Non-exclusive examples of reference beamrouting components include a mirror (653), a prism, split wedges, and anoptical fiber.

Routing the reference beam to illuminate the layer of holographicmaterial (510, 620) may include routing the reference beam to illuminatea first surface of the layer of holographic material (510, 620). Routingthe object beam and the reference beam to the same surface of the layerof holographic material (510, 620) allows recording of a transmissionhologram. Routing the reference beam to illuminate the layer ofholographic material (510, 620) may include routing the reference beamto illuminate a second surface of the layer of holographic material(510, 620). The second surface of the layer of holographic material(510, 620) is opposite the first surface of the layer of holographicmaterial (510, 620). Routing the object beam and the reference beam toopposite surfaces of the layer of holographic material (510, 620) allowsrecording of a reflection hologram.

At 808, the reference beam (642) is shaped to a desired cross-section atthe layer of holographic material (500). Non-exclusive examples ofreference beam shaping components include a lens (655) and a diffractiveoptical element.

At 809, a pattern of optical fringes is generated in at least at least aportion of the layer of holographic material (510, 620) by a combinationof the reference beam and the object beam. The layer of holographicmaterial (510, 620) may comprise a masked layer of holographic material(500). If the layer of holographic material (510, 620) comprises amasked layer of holographic material (500), the pattern of opticalfringes is generated only in the portion of the layer of holographicmaterial (510, 620) covered by the at least one permissive area (531) ofthe mask.

Generating a pattern of optical fringes in at least a portion of thelayer of holographic material (510, 620) by a combination of thereference beam and the object beam may include generating N sub-patternsof optical fringes in at least a portion of the layer of holographicmaterial by the combination of the reference beam and the object beam,where N is an integer greater than 1.

At 810, the pattern of optical fringes is recorded as a pattern ofphysical fringes in at least a portion of the layer of holographicmaterial (510, 620). If the layer of holographic material (510, 620)comprises a masked layer of holographic material (500), the pattern ofoptical fringes is recorded as a pattern of physical fringes only in theportion of the layer of holographic material (510, 620) covered by theat least one permissive area (531) of the mask.

Recording a pattern of optical fringes as a pattern of physical fringesin at least a portion of the layer of holographic material (510, 620)may include recording N sub-patterns of optical fringes as Nsub-patterns of physical fringes in at least a portion of the layer ofholographic material, where N is an integer greater than 1.

At 811, the mask (520) is removed from the layer of holographic material(510, 620).

At 812, the layer of holographic material is bleached. Bleaching thelayer of holographic material (510, 620) may include photobleaching.Method 800 may further comprise pre-bleaching the layer of holographicmaterial (510, 620). Pre-bleaching the layer of holographic material(510, 620) may include photo-bleaching the layer of holographic material(510, 620).

Pre-bleaching the layer of holographic material (510, 620) occurs priorto removing the mask (520) from the layer of holographic material (510,620). If the layer of holographic material (510, 620) is pre-bleached,applying a mask (520) to the layer of holographic material (510, 620)includes applying a negative mask to the layer of holographic material(510, 620). A negative mask comprises a mask (520) wherein theobstructive areas (532) cover the portion of the layer of holographicmaterial that will contain a recorded hologram. A negative maskcomprises a mask (520) wherein the at least one permissive area (531)covers the portion of the layer of holographic material that will notcontain a recorded hologram. Pre-bleaching the layer of holographicmaterial (510, 620) covered by a negative mask eliminates the need forapertures in the aperture-free hologram recording assembly (600).

Generating a laser light signal may include generating a laser lightsignal comprising M angles, where M is an integer greater than 1.Generating a pattern of optical fringes in at least a portion of thelayer of holographic material by a combination of the reference beam andthe object beam may include generating M sub-patterns of optical fringesin at least a portion of the layer of holographic material by thecombination of the reference beam and the object beam. Recording thepattern of optical fringes as a pattern of physical fringes in at leasta portion of the layer of holographic material may include recording theM sub-patterns of optical fringes as M sub-patterns of physical fringesin at least a portion of the layer of holographic material. Recordingthe M sub-patterns of optical fringes as M sub-patterns of physicalfringes records an angle-multiplexed hologram in the layer ofholographic material (510, 620).

FIG. 9 is a flow-diagram showing a method 900 of recording a hologram inaccordance with the present systems, devices, and methods. Method 900includes twelve acts 901, 902, 903, 904, 905, 906, 907, 908, 909, 910,911, and 912 though those of skill in the art will appreciate that inalternative embodiments certain acts may be omitted and/or additionalacts may be added. Those of skill in the art will also appreciate thatthe illustrated order of the acts is shown for exemplary purposes onlyand may change in alternative embodiments.

As an illustrative example of the physical elements of method 900,analogous structures from FIG. 5 and FIG. 6 are called out inparentheses throughout the description of acts 901, 902, 903, 904, 905,906, 907, 908, 909, 910, 911, and 912.

At 901, a layer of holographic material (510, 620) is mounted in anaperture-free recording assembly (600). The aperture-free hologramrecording assembly (600) comprises a laser light source (670) and anaperture-free optical path (610). The laser light source (670) does notrequire an aperture in order to generate a laser light signal, the beamdiameter of the laser light signal may be controlled via careful designof a resonant chamber of the laser light source (670). The laser lightsource may comprise at least one aperture. The aperture-free opticalpath (610) may comprise beam-routing mirrors (651, 652, 653), and theaperture-free optical path (610) may comprise beam-shaping lenses (654,655), however the aperture-free optical path (610) does not comprise anyapertures. The aperture-free optical path (610) does not include thelaser light source (670).

At 902, a mask (520) is applied to the layer of holographic material(510, 620). The mask (520) comprises at least one obstructive areawherein the at least one obstructive area is configured to shield aportion of the layer of holographic material from light exposure. Themask (520) comprises at least one permissive area wherein the at leastone permissive area is configured to expose a portion of the layer ofholographic material to light. The mask (520) eliminates the need forapertures in the aperture-free hologram recording assembly (600).

At 903, a laser light signal is generated with at least one laser lightsource (670). Generating a laser light signal may include generating alaser light signal with a laser light source (670), wherein the laserlight source (670) comprises an aperture.

At 904, the laser light signal is split with at least one beamsplitter(630) to form N object beams (641) and M reference beams (642), where Nand M are both integers that are greater than or equal to 1.Non-exclusive examples of beamsplitters include a beamsplitter cube, aWollaston prism, and a semi-silvered mirror.

At 905, the N object beams (641) are routed to illuminate the layer ofholographic material (510, 620). Non-exclusive examples of object beamrouting components include a mirror (651, 652), a prism, split wedges,and an optical fiber. Routing the N object beams to illuminate the layerof holographic material (510, 620) may include routing the object beamto illuminate a first surface of the layer of holographic material (510,620).

At 906, the N object beams (641) are shaped to N respectivecross-sections at the layer of holographic material (510, 620).Non-exclusive examples of object beam shaping components include a lens(654) and a diffractive optical element.

At 907, the M reference beams (642) are routed to illuminate the layerof holographic material (500). Non-exclusive examples of reference beamrouting components include a mirror (653), a prism, split wedges, and anoptical fiber.

Routing the M reference beams (642) to illuminate the layer ofholographic material (510, 620) may include routing the M referencebeams (642) to illuminate a first surface of the layer of holographicmaterial (510, 620). Routing the N object beams and the M referencebeams to the same surface of the layer of holographic material (510,620) allows recording of a transmission hologram. Routing the Mreference beams to illuminate the layer of holographic material (510,620) may include routing the M reference beams to illuminate a secondsurface of the layer of holographic material (510, 620). The secondsurface of the layer of holographic material (510, 620) is opposite thefirst surface of the layer of holographic material (510, 620). Routingthe N object beams and the M reference beams to opposite surfaces of thelayer of holographic material (510, 620) allows recording of areflection hologram.

At 908, the M reference beams (642) are shaped to M respectivecross-sections at the layer of holographic material (510, 620).Non-exclusive examples of reference beam shaping components include alens (655) and a diffractive optical element.

At 909, a pattern of optical fringes is generated in at least a portionof the layer of holographic material (510, 620) by a combination of theM reference beams and the N object beams. The layer of holographicmaterial (510, 620) may comprise a masked layer of holographic material(500). If the layer of holographic material (510, 620) comprises amasked layer of holographic material (500), the pattern of opticalfringes is generated only in the portion of the layer of holographicmaterial (510, 620) covered by the at least one permissive area (531) ofthe mask.

At 910, the pattern of optical fringes is recorded as a pattern ofphysical fringes in at least a portion of the layer of holographicmaterial (500). If the layer of holographic material (510, 620)comprises a masked layer of holographic material (500), the pattern ofoptical fringes is recorded as a pattern of physical fringes only in theportion of the layer of holographic material (510, 620) covered by theat least one permissive area (531) of the mask.

Generating a pattern of optical fringes in at least a portion of thelayer of holographic material (510, 620) by a combination of thereference beam and the object beam may include generating L sub-patternsof optical fringes in at least a portion of the layer of holographicmaterial by the combination of the reference beam and the object beam,where L is an integer greater than 1.

At 911, the mask is removed from the layer of holographic material (510,620).

At 912, the layer of holographic material is bleached. Bleaching thelayer of holographic material (510, 620) may include photobleaching.Method 900 may further comprise pre-bleaching the layer of holographicmaterial (510, 620). Pre-bleaching the layer of holographic material(510, 620) may include photo-bleaching the layer of holographic material(510, 620).

Pre-bleaching the layer of holographic material (510, 620) occurs priorto removing the mask (520) from the layer of holographic material (510,620). If the layer of holographic material (510, 620) is pre-bleached,applying a mask (520) to the layer of holographic material (510, 620)includes applying a negative mask to the layer of holographic material(510, 620). A negative mask comprises a mask (520) wherein theobstructive areas (532) cover the portion of the layer of holographicmaterial that will contain a recorded hologram. A negative maskcomprises a mask (520) wherein the at least one permissive area (531)covers the portion of the layer of holographic material that will notcontain a recorded hologram. Pre-bleaching the layer of holographicmaterial (510, 620) covered by a negative mask eliminates the need forapertures in the aperture-free hologram recording assembly (600).

Generating a laser light signal may include generating a laser lightsignal comprising L wavelengths of laser light, where L is an integergreater than 1. Generating a pattern of optical fringes in at least aportion of the layer of holographic material by a combination of thereference beam and the object beam may include generating L sub-patternsof optical fringes in at least a portion of the layer of holographicmaterial by the combination of the reference beam and the object beam.Recording the pattern of optical fringes as a pattern of physicalfringes in at least a portion of the layer of holographic material mayinclude recording the L sub-patterns of optical fringes as Lsub-patterns of physical fringes in at least a portion of the layer ofholographic material. Recording the L sub-patterns of optical fringes asN sub-patterns of physical fringes records an angle-multiplexed hologramin the layer of holographic material (510, 620).

FIG. 10 is a top elevational view of curved HOE 1000 in accordance withthe present systems, devices, and methods. Curved HOE 1000 comprises asingle contiguous layer of photopolymer material. Curved HOE 1000 may besubstantively similar to HOE 200. Curved HOE comprises eye-side surface1010 and world-side surface 1020. Curved HOE 1000 is cylindricallycurved around an axis of curvature, the axis of curvature of HOE 200 islocated on the eye-side of curved HOE 1000 at a distance of between 1and 10 centimeters, between 10 and 50 cm, or between 50 and 100 cm fromeye-side surface 1010.

Throughout this specification and the appended claims, infinitive verbforms are often used. Examples include, without limitation: “to detect,”“to provide,” “to transmit,” “to communicate,” “to process,” “to route,”and the like. Unless the specific context requires otherwise, suchinfinitive verb forms are used in an open, inclusive sense, that is as“to, at least, detect,” to, at least, provide,” “to, at least,transmit,” and so on.

The above description of illustrated embodiments, including what isdescribed in the Abstract, is not intended to be exhaustive or to limitthe embodiments to the precise forms disclosed. Although specificembodiments of and examples are described herein for illustrativepurposes, various equivalent modifications can be made without departingfrom the spirit and scope of the disclosure, as will be recognized bythose skilled in the relevant art. The teachings provided herein of thevarious embodiments can be applied to other portable and/or wearableelectronic devices, not necessarily the exemplary wearable electronicdevices generally described above.

The various embodiments described above can be combined to providefurther embodiments. To the extent that they are not inconsistent withthe specific teachings and definitions herein, all of the U.S. patents,U.S. patent application publications, U.S. patent applications, foreignpatents, foreign patent applications and non-patent publicationsreferred to in this specification and/or listed in the Application DataSheet which are owned by Thalmic Labs Inc., including but not limitedto: US Patent Application Publication No. US 2017-0068095 A1, US PatentApplication Publication No. US 2017-0212290 A1, U.S. Provisional PatentApplication Ser. No. 62/487,303, U.S. Provisional Patent ApplicationSer. No. 62/534,099, U.S. Provisional Patent Application Ser. No.62/565,677, U.S. Provisional Patent Application Ser. No. 62/482,062, andU.S. Provisional Patent Application Ser. No. 62/593,073 are incorporatedherein by reference, in their entirety. Aspects of the embodiments canbe modified, if necessary, to employ systems, circuits and concepts ofthe various patents, applications and publications to provide yetfurther embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A method of fabricating a holographic recording medium (“HRM”), themethod comprising: applying a mask to a layer of holographic material,the mask comprising: at least one obstructive area wherein the at leastone obstructive area is configured to shield a portion of the layer ofholographic material from light exposure; and at least one permissivearea wherein the at least one permissive area is configured to expose aportion of the layer of holographic material to light; bleaching themasked layer of holographic material, wherein bleaching the masked layerof holographic material includes exposing the at least one permissivearea of the mask to light; and removing the mask from the layer ofholographic material.
 2. The method of claim 1 wherein the layer ofholographic material comprises a front surface and a back surface, andwherein applying a mask to the layer of holographic material includes:applying a front mask to the front surface of the layer of holographicmaterial, wherein the front mask comprises at least one permissive areaand at least one obstructive area; and applying a back mask to the backsurface of the layer of holographic material, wherein the back maskcomprises a single obstructive area.
 3. The method of claim 1 whereinapplying a mask to a layer of holographic material includes applying amask to a layer of holographic material wherein the mask includes atleast one obstructive area with a shape selected from a group consistingof: a circle, an oval, a triangle, a square, a rectangle, a hexagon, andan octagon.